This invention relates to metallic implants with load bearing surfaces coated with a thin, dense, low friction, highly wear-resistant, uniformly thick coating of oxidized zirconium. This invention also relates generally to metallic implants with load bearing, abrasion resistant surfaces. In the present invention, the load bearing oxidized zirconium surfaces or abrasion resistant surfaces contact counter bearing surfaces of cross-linked polyethylene (XLPE). XLPE has superior wear characteristics compared with other conventional polymer materials used in prostheses. Oxidized zirconium has thermal conductivity characteristics that are particularly advantageous when used in a prosthetic device in which it articulates against XLPE. The unique advantages of oxidized zirconium and abrasion resistant surfaces in combination with those of XLPE result in a synergy which allows one to accentuate the superior properties of XLPE as a counter bearing surface, resulting in a superior prosthetic device.
Historically prostheses of articulating surfaces were constructed of materials of differing hardness for the contacting surfaces. By having one “yielding” surface, such prior art devices eventually form an optimal fit, i.e., a tight tolerance, whereby galling, fretting, and other erosive phenomena are minimized, resulting in longer-lasting prosthetic devices. An example of these early-generation devices is the femoral head of a hip-stem prosthesis which engages a counter-bearing surface in an acetabular cup which is often made of a softer material such as ultra-high molecular weight polyethylene. However, use of contacting surfaces of different hardness is not a perfect solution. The softer surface is, by nature sacrificial; it will eventually fail, its main virtue is the realization of an overall increase in the useful life of the prostheses. Additionally, fretting of the softer surface results in debris that may have deleterious effects on the health on the patient.
The invention described herein is a particular type of ceramic-on-polymer prosthesis. Its unique compositional properties affords the traditional advantages of ceramic-on-polymer systems while avoiding their major disadvantage.
The invention overcomes the major disadvantage generally inherent in prosthetic devices having hard surfaces articulating against soft surfaces. The basic technology upon which the improvement described herein is based, is described in U.S. Pat. No. 5,037,438 to Davidson and U.S. Pat. No. 6,447,550 to Hunter, et al., both of which are fully incorporated by reference herein.
The longevity of medical implant devices is of prime importance as it is desirable that the implant should function for the complete lifetime of a patient. This is particularly true if the patient is young and the number of surgical revisions is to be kept to a minimum and preferably zero. To this end, orthopedic implant materials should preferably combine high strength, corrosion resistance and tissue compatibility. One of the variables affecting the longevity of load-bearing implants such as hip-joint implants is the rate of wear of the articulating surfaces and long-term effects of metal ion release. A typical hip-joint prosthesis includes a stem, a femoral head and an acetabular cup against which the femoral head articulates. Wear of either or both of the articulating surfaces results in an increasing level of wear particulates and “play” between the femoral head and the cup against which it articulates. Wear debris can contribute to adverse tissue reaction leading to bone resorption, and ultimately the joint must be replaced.
The rate of wear of the acetabular cup and the femoral head surfaces of artificial hips is dependent upon a number of factors which include the relative hardness and surface finish of the materials which constitute the femoral head and the acetabular cup, the frictional coefficient between the materials of the cup and head, the load applied and the stresses generated at the articulating surfaces. The most common material combinations currently used in the fabrication of hip-joint implants include femoral heads of cobalt, titanium, or zirconium alloys articulating against acetabular cups lined with organic polymers or composites of such polymers including, for instance, ultra-high molecular weight polyethylene (UHMWPE) and femoral heads of polished alumina in combination with acetabular cups lined with an organic polymer or composite or made of polished alumina.
Of the factors which influence the rate of wear of conventional hip-joint implants, the most significant are patient weight and activity level. Additionally, heat generated by friction in the normal use of the implant has been shown to cause accelerated creep and wear of the polyethylene cup. Furthermore, there is a correlation between the frictional moment which transfers torque loading to the cup and the frictional coefficient between the femoral head and the surface of the acetabular cup against which the head articulates. Cup torque has been associated with cup loosening. Thus, in general, the higher the coefficient of friction for a given load, the higher the level of torque generated. Ceramic bearing surfaces have been shown to produce significantly lower levels of frictional torque. It is also noteworthy that two of the three commonly used hip-joint systems as indicated above include a metallic femoral head articulating against an ultra high molecular weight polyethylene (UHMWPE) liner inside the acetabular cup. UHMWPE, being a polymeric material, is more susceptible to creep at higher temperatures than the commonly used metal alloys or ceramics due to its relatively lower melting point and is consequently more susceptible to wear than the alloys or ceramics.
The original impetus for the inclusion of surfaces such as UHMWPE was that they would act sacrificially; they would fail slowly and fail before the harder surface, allowing for an overall extension of the useful life of the device. Additionally, polyethylene was thought to absorb shock much better than harder surfaces, thereby simulating the effect of real cartilage. While the advance in the art which was realized by the use of oxidized zirconium surfaces articulating against UHMWPE surfaces was a lessening of wear and cup loosening between the surface of the metallic component and the UHMWPE, the problem was not completely eliminated. Thus, the instant invention represents another advancement in the art, namely, a further improvement in wear and a simultaneous significant improvement of the creep problem associated with the prior art prostheses comprising polyethylene articulating against harder surfaces.
It has also been found that metal prostheses are not completely inert in the body. Body fluids act upon the metals causing them to slowly corrode by an ionization process thereby releasing metal ions into the body. Metal ion release from the prosthesis is also related to the articulation and rate of wear of load bearing surfaces because, as may be expected, when a metallic femoral head, for instance, is articulated against UHMWPE, the passive oxide film which forms on the femoral head is constantly removed. The repassivation process constantly releases metal ions during this process. Furthermore, the presence of third-body wear (cement or bone debris) accelerates this process and micro-fretted metal particles can increase friction. Consequently, the UHMWPE liner inside the acetabular cup, against which the femoral head articulates, is subjected to accelerated levels of creep, wear, and torque. A reduction in these deleterious effects will also improve the problem of metal ion release.
A number of attempts to correct these problems were the subject of much of the early work in this area. U.S. Pat. No. 4,145,764 to Suzuki taught a metal prosthesis plasma sprayed with a bonding agent which is in turn covered with a porous ceramic coating which would allow the in-growth of bone spicules into the pores. However, the Suzuki patent did not address the issue of friction or wear of orthopedic implant bearing surfaces but confined itself to the single issue of the biocompatibility of metal prostheses and did not address the issue of dimensional changes that occur when applying such a coating. U.S. Pat. No. 3,677,795 to Bokros is directed to the application of a carbide coating over a metallic prosthetic device. The method is said to produce a prosthetic device which has “excellent compatibility with body tissue and is non-thrombogenic”. However, Bokros does not address the issues of friction, heating, creep and wear of orthopedic implant bearing surfaces, or changes induced in the mechanical properties of the underlying metal due to this high temperature treatment.
The aforementioned failings of the prior art were addressed in part by Davidson in U.S. Pat. No. 5,037,438. In the '438 patent, Davidson teaches a zirconium or zirconium-containing metal alloy prosthesis coated via in-situ oxidation with a surface of blue-black or black oxidized zirconium which articulates against an organic polymer or polymer-based composite. The oxidized zirconium coating provides the prosthesis with a thin, dense, low friction, wear resistant, biocompatible surface ideally suited for use on articulating surfaces of joint prostheses wherein a surface or surfaces of the joint articulates, translates or rotates against mating joint surfaces. The oxidized zirconium coating described the '438 patent may therefore be usefully employed on the femoral heads or inside surfaces of acetabular cups of hip-joint implants or on the articulating surfaces of other types of prostheses, such as knee joints. Notably, the oxidized zirconium coating of the '438 patent was a specific type of oxidized zirconium. Oxidized zirconium presents itself in many forms, among them are white, beige, and blue-black. The white variety is particularly disfavored in the present application, as it tends to separate and break off of the substrate. Conventional oxidized zirconium surfaces formed, for example, by simple air oxidation will not be of the blue-black or black variety and will not possess the superior properties of the same which are recited in the '438 patent. The most important of these properties high hardness, low friction that results from the presence of the surface oxide.
The specific blue-black or black oxidized zirconium coatings of the '438 patent were known in the art of mechanical bearings, having been described in U.S. Pat. No. 2,987,352, which teaches a 700–1100° F. oxidation method to produce the specific blue-black or blue oxidized zirconium coating. A later issuing patent to Haygarth (U.S. Pat. No. 4,671,824) teaches an alternative, salt-bath method to produce the desired coating. The blue-black or black oxidized zirconium of the instant invention possessing the necessary properties is primarily monoclinic crystal structure. This has been characterized by Hunter et al. (Hunter, G, et al., Mat. Res. Symp. Proc., (1999), 550, 337).
The introduction of XLPE as a counter bearing surface in articulating prostheses was another unrelated attempt to address the problem of the relatively short service life of the UHMWPE component. Cross-linking of UHMWPE forms covalent bonds between polymer chains which retard the process of wear through the internal reinforcement of the individual polymer chains. However, XLPE is not without problems. The advantages of XLPE over other forms of polyethylene diminish as the roughness of the counter bearing surface increases and the operating temperature of device increases. Thus, a counter bearing surface, which possesses properties that prevent or improve the aforementioned conditions, would accentuate the advantages of XLPE over conventional forms of polyethylene. Additionally, there is anecdotal evidence that the improvement of wear characteristics comes at a cost of greater susceptibility to creep, particularly at elevated temperatures.
Another prosthetic device which benefits from the novel oxidized zirconium/XLPE couple described herein are vertebral disc implants. These devices are an increasingly popular alternative to the spinal fusion surgical procedure. This alternative typically results in the maintenance of range of motion at the operative level once a damaged disc has been replaced with an implant. One type of vertebral disc implant, described in U.S. Pat. No. 5,410,269 is that with two terminal plates which are connected to the endplates of the relevant vertebrae, and with a prosthesis core which cooperates with at least one terminal plate via an articular surface which permits a pivoting movement. Typically, the core material of these implants is high density polyethyelene, while the opposing plates are made of materials commonly found in conventional prosthetic devices, such as ceramic or metal. The use of disc implants having XLPE articulating against oxidized zirconium surfaces will not only decrease wear and improve the useful life of the device, but will also better maintain motion at the operative level upon removal of the damaged disc, relative to that of vertebral disc implants fabricated from conventional materials.
XLPE devices have exhibited other deficiencies, with which the prior art has largely been concerned about. Free radicals formed during irradiation, however, can exist indefinitely if termination by cross-linking or other forms of recombination do not occur. Furthermore, reacted intermediates are continuously formed and decayed. Exposure of these free radical species at any time (e.g., during irradiation, shelf-aging, or in vivo aging) to molecular oxygen or any other reactive oxidizing agent can result in their oxidation. Extensive oxidation leads to a reduction in molecular weight, and subsequent changes in physical properties, including wear resistance. Many attempts have been made to improve the characteristics of XLPE. These attempts include radiation induced cross linked polyethylene (See U.S. Pat. Nos. 5,728,748; 5,650,485; 5,543,471; 5,414,049; and 5,449,745 to Howmedica; Johnson & Johnson's EP 0737481 A1, see also Hamilton, J. V. et al., Scientific Exhibit, 64th AAOS Meeting, February 1997; Hamilton, J. V. et al., Trans 43rd ORS, 782, 1997; Biomet's World Patent Application No. 97/29787; see also Oonishi, H. et al., Radiat. Phys. Chem., 39(6), 495, 1992; Oonishi, H. et al., Mat. Sci: Materials in Medicine, 7, 753–63, 1966, Oonishi, H. et al., J. Mat. Sci: Materials in Medicine, 8, 11–18, 1997; U.S. Pat. No. 5,879,400; World Patent Application WO 98/01085; U.S. Pat. No. 6,165,220; EP 0729981 A1; U.S. Pat. No. 6,017,975; and U.S. Pat. No. 6,228,900, for chemical cross-linking of polyethylene (See EP 0722973 A1)).
In the present invention, the improvement in the performance of XLPE is realized not through improvements in the XLPE composition itself, but rather through the use of oxidized zirconium or other abrasion resistant surface as a counter bearing surface against which the XLPE component articulates. The advantages of the instant invention will be the preservation of the desirable properties of XLPE with a simultaneous elimination of some of its negative properties. The '438 patent did not contemplate and nowhere does it teach, the use of the oxidized zirconium surfaces directly contacting surfaces of XLPE. The inventors have discovered that the unique properties of oxidized zirconium accentuate the inherent advantages of XLPE as a counter bearing surface. The superior strength and hardness, low friction, wear resistance, thermal conductivity, and biocompatibility characteristics of the blue-black or black oxidized zirconium is sufficient in itself to considerably slow and possibly prevent the degradative wear processes to which the prosthetic devices of the prior art have been subject. An unapparent synergy is realized because the unique properties of oxidized zirconium serve to improve the performance of XLPE as a counter bearing surface. These unexpected advantages are also present, to a lesser degree, when other abrasion resistant surfaces are used.
The invention is directed to forming prosthetic devices of oxidized zirconium-on-XLPE, which represents a special species of oxidized zirconium-on-polymer devices, exhibiting even longer overall useful service life relative to conventional prostheses materials-on-UHMWPE. This is due not only to the advantages which inure upon the substitution of oxidized zirconium for conventional prosthesis materials, but also from the synergistic improvement in XLPE performance that is seen when XLPE articulates against oxidized zirconium. The invention is not limited to prostheses formed of zirconium or zirconium alloy. The prostheses substrate may itself be a composite material, only requiring that zirconium or zirconium alloy be present in the substrate layer immediately adjacent to the surface upon which the oxidized zirconium coating is to be formed.